1. Field of the Invention
The present invention relates to crossed-field devices such as magnetrons having a radial electric field and an axial magnetic field, and more particularly, to a positive anode magnetron in which a tertiary electric field is utilized to enable improved frequency tuning ability.
2. Description of Related Art
Magnetrons are a type of crossed-field device that is commonly used to generate high power microwave energy for assorted applications, such as radar. A magnetron typically comprises a cylindrically shaped cathode that extends axially along a central axis of an anode structure comprising a plurality of radially extending anode vanes. A space defined between the cathode surface and the tips of the anode vanes provides an interaction region, and an electric potential is applied between the cathode and the anode forming a radial electric field in the interaction region. An axial magnetic field is provided in a direction perpendicular to the electric field and is directed to the interaction region by polepieces which focus magnetic flux from permanent magnets disposed externally of the interaction region. The cathode may be provided with an internal heater disposed below the surface of the cathode to heat the cathode surface to a temperature sufficient to cause thermionic emission of electrons therefrom. The emitted electrons are caused to orbit around the cathode in the interaction region due to the axial magnetic field, during which they interact with an electromagnetic wave that is caused to move on the anode structure. The orbiting electrons give off energy to the electromagnetic wave, thus resulting in a high power microwave output signal.
In conventional magnetrons, the polepieces and the anode structure are disposed at the same electric potential relative to the cathode. The magnetron may be operated with negative voltage pulses being applied to the cathode with the anode grounded, or with positive pulses being applied to the anode with the cathode grounded. An advantage of such a magnetron structure is that the polepieces need not be isolated electrically from the anode, and thus, construction of the device is simplified. On the other hand, the potential difference between the cathode and the polepieces causes undesirable spreading, or defocusing, of the electron stream that extends between the cathode and the anode structure. As known in the art, the electron stream may be refocused to counteract the spreading phenomenon by tapering the polepieces to distort the magnetic field and by disposing end hats on the cathode to distort the electric field.
An alternative structure which tends to avoid the electron stream spreading phenomenon described above is known as the positive anode magnetron. In a positive anode magnetron, the cathode and the polepieces are disposed at the same electric potential relative to the anode. As a result, the electron stream remains focused as it passes between the cathode and the anode structure, thus eliminating the need to distort the magnetic field. An example of a positive anode magnetron is provided by U.S. Pat. No. 4,104,559, to Hobbs, for ISOPOLAR MAGNETRON SUPPORTED WITH RIGID INSULATION IN A REMOTE HOUSING.
A problem common to each of the known magnetron types is that of efficiently tuning the device to alter its operating frequency. There are three known methods of tuning a magnetron. A first method is to mechanically alter the resonant structure of the magnetron to alter its inductance or capacitance, and thereby change the output frequency. For example, the volume of the structure may be changed by a movable end plate or other such member. A significant drawback of this first method is that the response time of any such alterations is limited by the mechanical mass of the movable member which restricts the amount that the frequency of the magnetron can be changed during the period between successive pulses.
A second method of tuning a magnetron is to place electrically active devices within the resonant structure of the magnetron, such as pin diodes or multipactor discharge gaps. Such active devices are limited to non-linear tuning, and moreover, they increase the cost and complexity of manufacturing. For example, pin diodes are extremely fragile and prone to breakage during the manufacturing process, and they dissipate significant power when used in other than a switching mode for frequency modulations. Multipactor devices use secondary emission surfaces which require special processing techniques.
A third method of tuning a magnetron is to insert a tunable structure into the high electric field region of the resonant structure. For example, a mechanical gear arrangement may be used to control the position of a rotatable tuning element disposed within the interaction region. The tuning element may comprise a dielectric structure that alters the magnetron frequency in relation to its alignment with the electric field lines. A drawback of this third tuning method is that it is only effective with crossed-field devices having a low quality factor, Q, or the ratio of energy stored to energy lost due to dissipation, and thus is not practical with high power magnetrons.
Accordingly, it would be desirable to provide a system for tuning a high-power crossed-field device which overcomes these and other substantial drawbacks of the prior art tuning systems. In particular, it would be desirable to provide such an improved tuning system for use with a positive anode magnetron.